The present invention relates to communications apparatus and methods, more particularly, to apparatus and methods for determining a direction of a radio signal.
Wireless communications systems such as cellular radiotelephone systems typically are subject to performance-degrading environmental effects such as multi-path fading and interference. Fading effects include flat fading, arising from the interaction of a transmitted signal (the main ray) with reflected versions of the transmitted signal that arrive concurrently at a receiver. Time dispersion, another type of fading, arises from interaction of the main ray with time-delayed reflections of the main ray. Interference effects may be caused by interaction of non-orthogonal signals generated in the signal medium by sources other than the source of the desired transmitted signal. Well-known techniques for reducing the effects of flat fading include diversity combining of signals from spatially separated antennas. Equalization techniques such as maximum likelihood sequence estimation (MLSE) may be used to compensate for time dispersion. Interference may be reduced by using antenna beam steering to reduce reception of undesired signals.
U.S. patent application Ser. No. 08/284,775, filed Aug. 2, 1994, assigned to the assignee of the present invention and incorporated herein by reference, has proposed an interference rejection combining (IRC) technique which compensates for flat fading, time dispersion and interference. An impairment autocorrelation matrix is computed and used to compute branch metrics for a sequence estimation process which are corrected for the presence of interfering signals, i.e., impairments, using the autocorrelation properties of the interfering signals. According to U.S. patent application Ser. No. 08/655,930, filed May 31, 1996, assigned to the assignee of the present application and incorporated herein by reference in its entirety, IRC may be used to determine a covariance matrix of a desired signal, and downlink beamsteering techniques may be employed based on the determined covariance to improve communications between a mobile unit and a base station
Unfortunately, however, estimates of direction of arrival of a desired signal produced using the approach described in the aforementioned U.S. patent application Ser. No. 08/655,930 can be inaccurate due to noise introduced in determining the covariance matrix of the desired signal. The noise susceptibility may in turn require increased sampling to achieve a desired accuracy for the estimate of the direction of arrival. Accordingly, there is a need for improved techniques of direction estimation that are less susceptible to noise and can provide more accurate results without requiring undue sampling.
In light of the foregoing, it is an object of the present invention to provide apparatus and methods for determining a direction for a desired signal that are resistant to fading and interference effects.
It is another object of the present invention to provide apparatus and methods for determining a direction for a desired signal that can provide greater accuracy than conventional techniques.
It is yet another object of the present invention to provide apparatus and methods for determining direction of an interfering signal.
These and other objects, features and advantages are provided according to the present invention by apparatus and methods in which a direction for a desired radio signal arriving at a receiving station is determined from a plurality of estimated signal medium response coefficients for a ray of the desired radio signal. According to an aspect of the present invention, the estimated signal medium response coefficients for the ray are generated according to an interference rejection combining (IRC) process, an equalization process that compensates for interference. According to another aspect, a direction of arrival for a ray of the desired signal is determined and used to compute a power spectral density for the ray for a plurality of directions. A direction of arrival for the ray is then determined by determining a direction for which the power spectral density has a maximum value. By determining desired signal direction utilizing coefficients for a ray which are normally generated as part of an IRC process, a technique for determining signal direction is provided which is potentially more efficient than conventional techniques. In addition to desired signal direction, direction of arrival for an interfering signal can be efficiently determined using an impairment autocorrelation matrix that is also generated as part of the IRC process.
In particular, according to the present invention, a direction for a desired radio signal generated in a communications medium is determined. A plurality of radio signals is received from the communications medium, e.g., at a like plurality of antennas. A plurality of estimated signal medium response coefficients for a ray of the desired radio signal are generated from the plurality of received radio signals. A direction for the desired radio signal is determined from the plurality of estimated signal medium response coefficients for the ray of the desired radio signal. A direction for the desired radio signal may be determined by determining a direction of arrival for the ray from the corresponding plurality of estimated signal medium response coefficients for the ray, and then determining a direction for the desired radio signal from the determined direction of arrival for the ray.
According to an aspect of the present invention, the plurality of estimated signal medium response coefficients for the ray may be generated by generating a first plurality of estimated signal medium response coefficients. An impairment autocorrelation matrix is then generated from the plurality of received signals and the first plurality of estimated signal medium response coefficients. A branch metric is generated from the generated impairment autocorrelation matrix and the first plurality of estimated signal medium response coefficients, and a symbol estimate is generated from the generated branch metric. A second plurality of estimated signal medium response coefficients may then be generated from the plurality of received signals and the generated symbol estimate.
According to another aspect of the present invention, the plurality of estimated signal medium response coefficients may be generated by generating a series of pluralities of estimated signal medium response coefficients over a time interval, processing the series of pluralities of estimated signal medium response coefficients with a tracking filter to produce a plurality of predicted signal medium response coefficients, and determining a determining a direction for the desired radio signal from the plurality of predicted signal medium response coefficients for the ray of the desired radio signal. The desired radio signal may be formatted according to a slotted communications protocol defined according to a plurality of time slots, and the series of pluralities of estimated signal medium response coefficients may be generated over at least one of the plurality of time slots.
According to another tracking aspect, a plurality of impairment autocorrelation matrices may be estimated over a time interval, e.g., over a time slot or a plurality of time slots in a slotted communications system. The generated plurality of impairment autocorrelation matrices may then be processed with a tracking filter to produce a predicted impairment autocorrelation matrix. A branch metric may then be generated from the predicted impairment autocorrelation matrix, and used to generate an improved direction estimate.
According to yet another method aspect of the present invention, a direction of arrival for the ray is determined by determining an autocorrelation value for the ray of the desired radio signal. A plurality of power spectral density values may be then be determined for the ray at a like plurality of directions using the determined autocorrelation value. A direction of the plurality of directions having a maximum associated determined power spectral density value is then identified to thereby determine a direction of arrival for the ray. According to another tracking aspect, a plurality of directions of arrival for the ray may be determined over a time interval such as a time slot or plurality of time slots. The determined plurality of directions of arrival may be processed with a tracking filter to produce a predicted direction of arrival for the ray.
In other method aspects, a direction for an interfering radio signal is determined from the generated impairment autocorrelation matrix. A plurality of power spectral density values may be determined for the interfering radio signal at a like plurality of directions using the generated impairment autocorrelation matrix. A direction of the plurality of directions having a maximum associated determined power spectral density value may then be identified to thereby determine a direction for the interfering radio signal. The determined direction of arrival may be tracked over a time interval, such as a time slot or series of time slots.
An apparatus for receiving a desired radio signal generated in a communications medium includes means, responsive to the communications medium, for receiving a plurality of radio signals from the communications medium. Means are responsive to the means for receiving for generating a plurality of estimated signal medium response coefficients for a ray of the desired radio signal from the plurality of received radio signals. Means are also provided, responsive to the means for generating, for determining a direction for the desired radio signal from the plurality of estimated signal medium response coefficients for the ray of the desired radio signal. The means for determining a direction for the desired radio signal may comprise means for determining a direction of arrival for the ray from the corresponding plurality of estimated signal medium response coefficients for the ray, as well as means for determining a direction for the desired radio signal from the determined direction of arrival for the ray.
The means for generating a plurality of estimated signal medium response coefficients may include means for generating a first plurality of estimated signal medium response coefficients. Means are responsive to the means for generating a first plurality of estimated signal medium response coefficients, for generating an impairment autocorrelation matrix from the plurality of received signals and the first plurality of estimated signal medium response coefficients. Additional means are provided, responsive to the means for generating an impairment autocorrelation matrix, for generating a branch metric from the generated impairment autocorrelation matrix and the first plurality of estimated signal medium response coefficients. Means are also provided, responsive to the means for generating a branch metric, for generating a symbol estimate from the generated branch metric. Means for generating a second plurality of estimated signal medium response coefficients from the plurality of received signals and the generated symbol estimate are provided. The apparatus may further include means for determining a direction for an interfering radio signal of the plurality of radio signals from the generated impairment autocorrelation matrix.